Process for treating silicon alloy castings



I Patented Sept. 4, 1934 UNITED STATES- PATENT OFFICE PROCESS FORTREATING SILICON ALLOY CASTINGS York No Drawing. Application June 9,1933, Serial No. 675,105

4 Claims. (Cl. 148 21.5)

.The invention relates to a process for treating high silicon iron alloycastings which have a composition rendering them highly resistant tocorrosives and particularly chlorides, but which are 5 also normallyextremely brittle because of their composition in such manner that thecastings are much stronger and also more resistant to heat shock.

High silicon iron castings of approximately 14.35 per cent siliconconsist structurally of a chemical compound of iron silicide dissolvedin iron. Naturally, a solution of this type is subject to brittleness sohighly characteristic of intermetallic compounds. Under the microscope 5the alloy appears as a typical solid solution with equa-axed grains. Ifcarbon is introduced into the iron-silicon alloy, some of it combineswith the iron silicide to form a eutectoid mixture and if an excess ofcarbon exists, primary graphite is also present.

The rate of solidification, as well as the total amount of carbonpresent, has a marked influence upon the relative amounts anddistribution of the iron silicide, eutectoid and primary graph- 5 ite.For instance, heavy castings poured in green sand, of an alloyconsisting of 14.25 per cent-14.50 per cent silicon and about 0.80 percent carbon, will exhibit under the microscope a solid background ofwhite iron silicide traversed by sinuous lines or flakes of proeutecticor primary graphite.

If the rate of solidification is much greater, as in a thinner section,particles of iron silicide will be found imbedded in a matrix oftheeutectoid, with some primary graphite also in evidence. It is alsopossible in castings of heavy section, to develop both types of thesestructures within the same section, the former type appearing in thecenter with the latter variety near the edges. Such a duplex structureis prone to develop internal stresses of appreciable magnitude withinthe casting. Primary graphite, especially when it appears in the form offlakes, sets up planes of additional weakness in the material, so thatfor maximum strength the preferred structure should consist of asorbitic nature throughout.

The alloy which I use in my' castings is of the high silicon type, abovediscussed, containing about 14 per cent of silicon and a maximum of .90per cent carbon, to which is added about 4 per cent of molybdenum. Thepresence of the molybdenum materially increases the corrosion resistanceof the alloy by reinforcing the silicon. Molybdenum forms an oxide(M003), which is an acid anhydride, very similar to the oxide of silicon(SiOz) Due to these oxides, a compound film is built up on the surfaceof the casting which is very effective in protecting it from corrosivesand particularly from concentrated chlorides. Molybdenum also forms acompound with iron, known as iron molybdide. The molybdenum is thussimilar to silicon in several respects, and should, therefore, react ina complementary manner.

Heretofore, in making high silicon iron castings containing molybdenum,difiiculties have arisen due to the fact that the molydenum readilycombines with carbon, and the presence of the molybdenum carbides thusformed, further increases the brittleness of the alloy. As a result, itis practically impossible to obtain castings from the sand which arefree from cracks, and if free from cracks, the castings are materiallyweaker than the straight silicon iron alloys. I have found that byproper heat treatment, it is possible to overcomethe diflicultiesincident to iron silicon alloys containing molybdenum, and to producecastings which are not only superior in corrosion resistance, but whichalso have superior physical characteristics, as compared with theordinary high silicon iron alloys.

In practicing the process, the castings are removed from their molds,while still at a red heat, and placed in a furnace previously heated toa temperature above 800 deg. F. The temperature is then elevated to 1500deg. F., and held at this point for a period of from 15 to 30 hours. Theheat supply is then cut off and slow cooling resorted to in the furnaceto the point where the castings can be safely and conveniently handled.As a result of this treatment, the casting sections are of entirelyuniform structure. In order to obtain this structure, it is necessary tocool very slowly, especially through the lower critical temperature inthe neighborhood of 1100 deg. F. In other words, castings, when treatedas above set forth, are not subject to the variations in structure withchange in section, nor does the structure vary from outside to center ina given section, as is the case in plain iron silicon carbon alloys, asheretofore pointed out. Further, the castings, as produced by theimproved process, are more readily cast in intricate shapes, are muchstronger, and are more resistant to heat shock.

If the castings were allowed to cool to room temperature in the moldswithout the above heat treatment, the rate of cooling is so rapid thatthe transformation temperature is correspondingly reduced, similar tothat of steel when quenched. The alpha iron is thereby prevented bides.

(due to its rigid characteristics) from dissolving carbon combined withthe molybdenum. The slower rate of cooling which occurs in my process,elevates the transformation point to a value comparable with thetheoretical (1100 deg. F.), so that the precipitated carbides arelarger, and begin to take on the form of a troostitic or sorbiticformation, rather than the cementitic structure of the casting which iscooled directly in the mold.

The molybdenum carbide is graphitized by the soaking treatment above andbelow the critical temperature, and the result is a completelyhomogeneous structure similar to troosite or sorbite. That is, the ironmolybdide and iron silicide forms this type of structure with freecarbon without the existence of any molybdenum car- In this way, thedesirable properties of the alloy, due to the presence of molybdenum,with respect to corrosion resistance are obtained, without thedeleterious effects of molybdenum carbides, which would ordinarilymaterially increase the tendency of the alloy to crack and decrease thestrength and resistance to heat shock.

While the proportions of silicon molybdenum and carbon, as heretoforegiven, are the preferred ones, these proportions may vary over aconsiderable range, while still retaining in the alloy in a largedegree, the improved characteristics as above set forth, when the alloycastings are treated as heretofore described. These proportions may varywithin the following limits:

Silicon 7 to 18% Molybdenum .1 to 10% Carbon .25 to 1.5%

In the event that primary or excess graphite is present over and abovethat necessary for the eutectoid small amounts of nickel added to thealloy serve to control the type and distribution. The invention,therefore, contemplates the addition of nickel to the formula heretoforegiven, and this may range up to two per cent. The desirable eifect ofthe heat treatment heretofore described obtains equally well with thisaddition of nickel.

Vanadium or tungsten may be substituted for molybdenum, and theimprovement in the castings under the heat treatment described will besimilar, but the molybdenum ispreferred because thecorrosion resistanceis greater than where vanadium or tungsten are used. It will also beunderstood that the invention contemplates combinations of these metals,although the use of the molybdenum alone is best. When the metalstungsten and vanadium are substituted for the molybdenum, the quantityused is the same as stated for the molybdenum.

It will also be understood that the invention comprehends the use ofother elements in the alloys, in such relatively small quantities aswill not materially affect the character of the alloys. Small amounts ofsulphur, phosphorous, manganese and the like, which occur more or lessas impurities, will also be understood to be comprehended by thisinvention.

What I claim is:

1. A process for treating silicon alloy castings containing 7.5 to 18per cent of silicon, and which also include .1 to 10 per cent ofmolybdenum and .25 to 1.5 per cent of carbon, which consists in removingthe castings from their molds while at approximately a red heat, raisingthe temperature of such castings to about 1500 deg. F., permitting themto soak at such temperature, and then slowly cooling the castings tohandling temperature.

2. A process for treating high silicon iron alloy castings which include.1 to 10 per cent of molybdenum and .25 to 1.5 per cent of carbon, whichconsists in removing the castings from their molds while atapproximately a red heat, raising the temperature of such castings toabout 1500 deg. F., permitting them to soak at such temperature, andthen slowly cooling the castings to handling temperature.

3. A process for treating high silicon iron alloy castings which include.1 to 10 per cent of molybdenum and .1 to 3 per cent of metal belongingto the'group nickel and cobalt, and .25 to 1.5 per cent of carbon, whichconsists in removing the castings from their molds while atapproximately a red heat, raising the temperature of such castings toabout 1500 deg. F., permitting them to soak at such temperature, andthen slowly cooling the castings to handling temperature.

4. A process of treating silicon iron alloy castings containing about 14per cent silicon, and which also includes about 4 per cent of molybdenumand .9 per cent of carbon, which consists in removing the castings fromtheir molds while at approximately a red heat, raising the temperatureof such castings to about 1500 deg. F., permitting them to soak atsuch-temperature, and then slowly cooling the castings to handlingtemperature.

JAMES A. PARSONS.

